1. Field of the Invention
This invention relates to a discharge lamp and, more particularly, to an electronic ballast which operates the lamp with minimal electromagnetic emission during power switching within the ballast.
2. Description of the Related Art
The structure and operation of a discharge lamp, henceforth referred to as a "lamp" is generally well known. A lamp typically comprises a quartz tube filled with gas. The ambient within the tube is exposed to a pair of electrodes spaced at opposite ends of the tube. During times when current is passed between the electrode pair, the gas is excited to a plasma state which causes light emission as the gas is being excited.
A ballast serves as the primary control element for the electrode pair. Essentially, a ballast operates as a current regulator. The ballast provides sufficient energy to excite the gas between the pair of electrodes. Generally speaking, there are two types of ballast: an electronic ballast or a core-coil ballast. Regardless of its form, a ballast is used to limit current through the lamp and hence limit power applied to the lamp.
Lamps, and ballasts associated with lamps, are used in various commercial settings. For example, a lamp may be used in a luminaire, such as that shown in FIG. 1. Luminaire 10 depicts various fixtures surrounding a lamp and ballast. Popular fixtures include any element (e.g. reflector, lens. etc.) which bring about a desired illumination pattern. Accordingly, luminaire 10 comprises a ballast 12, lamp 14 and various optics 16. Optics 16 may include special lenses for focusing or dispersing a light output 18. The lens may include a colored filter selectively placed across the lens surface for masking portions of the light output into a desired pattern. The pattern may rotate in conjunction with possible movement of luminaire 10 to achieve an almost limitless emission display upon object 20. That display being discernible by an observer 22 located distant from object 20.
In order to control lamp 14 output and selectively pattern luminaire 10 output, a control unit 24 may be used. Control unit 24 comprises various electronic circuitry required to control the output from ballast 12 and optics 16. For example, control unit 24 can be configured to forward various control signals at a select frequency dictated by the operator. The application of those intervals might be mandated by the demands of studio and stage lighting. Control unit 24 thereby comprises electronic circuits which can be hardware, software or firmware modified to produce a control signal of alterable time duration and/or intensity.
FIG. 2 depicts an example of several components used in forming ballast 12. Included with many conventional ballast is a pre-regulator 28. Regulator 28 serves many functions. For example, regulator 28 includes circuitry which can correct a power factor input of the power input signal to limit line harmonics and allow a wide range of input voltages. If converter 30 is a DC-to-DC converter, than an AC power input must be regulated to DC by regulator 28. Accordingly, regulator 28 will involve a means of rectification.
Converter 30 derives regulated power from regulator 28 and, more importantly, control signals from control unit 24 (shown in FIG. 1). Accordingly converter 30 can be a pulsewidth modulated ("PWM") converter, a primary function of which is to modulate the regulated power input to the converter according to the control signal duration (i.e., width). The control signals can be regularly dispatched at a given duty cycle to, for example, dim converter 30 output. Alternatively, the control signals can be sent in an irregular pattern, or at lower frequencies, to present a discernible output.
Converter 30 output may require additional regulation or conversion. The additional conversion may be in the form of a commutation output bridge 32, of well know design. The purpose of bridge 32 is to alternate the direction of current flow which is typically required by this type of lamp. Ignitor 43 serves primarily to ignite (i.e., ionize) the gas between the electrodes of lamp 14 (shown in FIG. 1).
One of the most important challenges facing electronic ballast designers is to make the ballast electromagnetically compliant. This entails limiting electromagnetic emissions from converter 30 during switch transients. More specifically. electromagnetic radiation is produced by switch transitions within converter 30 in response to the control signals forwarded thereto.
FIG. 3 illustrates a conventional portion of converter 30, and FIGS. 4a-4e depict current and voltage derived from converter 30 during transient switch conditions. Converter 30, comprises a network of active and passive components coupled between an input terminal and an output terminal. The input terminal receives the regulated voltage V.sub.IN, and the modulated voltage is produced at the output terminal as V.sub.OUT. The regulator output can be modeled as having a large output impedance partially depicted as capacitor 40. Capacitor 40 maintains the power supply V.sub.IN, regardless of the state of switch 42. Switch 42 comprises any power switching device, such as a thyristor or transistor (MOS or bipolar) which is responsive to the control signal. During times when switch 42 is open, freewheeling diode 44 is forward biased and receives the current through inductor 46. The resistor 50 simulates a load. The load being that attributable to any device connected to converter 30, the device being, for example, a commutation device, ignitor or lamp.
During times when switch 42 is closed, diode 44 becomes reverse biased based on the translation of voltage V.sub.IN to node V.sub.A. Movement from a forward bias to a reverse bias condition does not instantaneously terminate current through diode 44. Instead, diode 44 will conduct current in a negative fashion due to what is known as the "reverse recovery" characteristics of the diode. Reverse recovery occurs only momentarily but, unfortunately, causes relatively large current transient in the interim between the forward bias and reverse bias condition. The peak magnitude of this current is increased by attention to printed wiring board layout for minimal electromagnetic radiation.
Reverse recovery is dictated from the natural response of a diode or rectifier pn junction. When a diode is driven from a reversed biased condition to a forward biased condition, the diode response is accompanied by a current transient for a time before it recovers to its steady state. The transient period from a reverse to a forward condition is known as forward recovery time. Transition time from a forward bias condition to a reverse bias condition is known as reverse recovery time, as described above. As a practical matter, the forward recovery time does not normally constitute a serious practical problem, and hence it is primarily the reverse recovery time that limits diode switching performance.
The larger reverse recovery time is dictated by the time it takes to sweep minority carriers originally derived during forward bias from the other side of the junction back into that junction during reverse bias. To attain steady state value, the minority carrier distribution at the moment of voltage reversal requires the injected, or excess minority carriers to drop nominally to zero. Until that happens, the diode will continue to conduct current in either direction across the pn interface.
The effect of reverse recovery on noise generation is shown in reference to FIGS. 4a-4e. The magnitude of the reverse recovery current is limited only by the stray inductance of loop 43. The contribution of loop 43 electromagnetic noise is proportional to the peak current. It is only during the transient portion of switch 42 operation that reverse recovery becomes a problem, that problem manifesting itself as noise emitted from converter 30 as well as power dissipated in the switch and diode and the voltage and current stresses applied to those components . The noise is generally termed electromagnetic ("EM") noise resulting primarily from current, voltage and power spikes during the switch transition interval.
FIG. 4a depicts the voltage at node VA. That voltage increases rather sharply at time T.sub.1 when switch 42 is closed. A certain amount of ringing appears during the interim immediately after switch 42 is closed at T.sub.1 and before voltage reaches steady state at T.sub.2. Ringing also occurs between time T.sub.3 and T.sub.4, represented as the interval between opening of switch 42 at time T.sub.3 and steady state at time T.sub.4. Thus, closure of switch 42 occurs at time T.sub.1, and opening of switch 42 occurs at time T.sub.3. Times T.sub.2 -T.sub.3 illustrate steady state.
FIG. 4b indicates current I.sub.Q1 through switch 42. I.sub.Q1 peaks at the interim between T.sub.1 and T.sub.2 due primarily to the reverse recovery current through diode 44. That peak does not, however, demonstrate itself at intervals between T.sub.3 and T.sub.4 due to the lessening problem of forward recovery current. Thus, the problem of EM noise occurs almost predominantly during closure of switch 42.
FIG. 4c indicates the reverse recovery current I.sub.D1 through diode 44 in the interim between T.sub.1 and T.sub.2. I.sub.D1 extends from the n side of the junction to the p side of the junction, and therefore is represented as a negative value from the arrow shown in FIG. 3.
FIG. 4d illustrates a somewhat consistent current I.sub.L1 through inductor 46 during closing and opening of switch 42. If switch 42 is opened, then power from V.sub.IN is terminated to inductor 46, and I.sub.L1 decreases. However, once switch 42 is closed, then the supplied power produces an increase in I.sub.L1. Transient current peaks through inductor 46 are minimized by the nature of inductance and also by the voltage clamping action of freewheeling diode 44 whenever switch 42 is open.
FIG. 4e illustrates the effect of reverse recovery current on power dissipation P.sub.Q1 through switch 42 during the transient period between T.sub.1 and T.sub.2. During the reverse recovery time immediately after switch 42 closure, diode 44 remains in conduction and has not yet established a depletion region. In this state, diode 44 will conduct in either direction until a junction has sufficient time to discharge the stored minority carriers. The current magnitude during the reverse recovery time (i.e., reverse recovery current) is typically quite high and is limited predominantly by the inductance within the trace conductor load as viewed by loop 43. Power dissipation principally arising from reverse recovery (i.e., reverse recovery power) is labeled in FIG. 4e as reference numeral 54.
It is desired that reverse recovery power, demonstrated as EM noise and heat, be minimized in many industrial lighting applications. For example, EM radiation from ballast used in stage illumination must be minimized so as not to effect for example, sound equipment placed nearby. Large EM radiation might interfere with sound reproduction characteristics of high fidelity instruments, amplifiers, audio/video products, computers, and radio frequency ("RF") devices, etc., placed near the noisy ballast. An improvement in conventional ballast design for certain discharge lamp applications is therefore needed to meet various compliance requirements.